A variable fee or margin is a percentage of the earned rewards that a pool operator keeps.
For example, suppose the GOAT pool earned 10000 ADA in the past epoch. The protocol automatically deducts 340 ADA as a fixed fee from the 10000 ADA and sends it to our wallet. Therefore 10000-340=9660 ADA is left.
Then variable fee will be calculated and deducted based on the percentage set by the stake pool operator. In the case of the GOAT pool, we charge 2.5%. Thus 9660 x 2.5%=241.5 ADA.
Delegators will be left with 9660-241.5=9418.5 ADA, distributed to each delegator in proportion to the amount of stake delegated.
Both the fixed fee and the variable fee help the operator cater to the costs of managing nodes on the Cardano network. These include internet connection fees, power bills, and labor.